Human bone marrow cells derived from multiple sclerosis (MS) and control patients were screened for a number of virus antigens by the fluorescent antibody technique using monoclonal antibodies. The results showed that antigens of the paramyxovirus, simian virus 5, were present in about 60% of MS and 25% of control bone marrows. About 25% of the MS and 50% of control bone marrows were found to contain nucleoprotein antigen of the human parainfluenza types 1 and 3. These experiments demonstrated that paramyxoviruses can persist in human tissues possibly in a defective or repressed state.